Multiple antenna transmission may be classified into two types, i.e. single-user multiple input multiple output (SU-MIMO) transmission mode and multiple-user multiple input multiple output (MU-MIMO) transmission mode. FIG. 1(A) illustrates the SU-MIMO transmission mode, and FIG. 1(B) illustrates the MU-MIMO transmission mode. As shown in FIG. 1(A), in SU-MIMO transmission mode, the base station (BS) transmits multiple data flows on the same time frequency resource to the same user. SU-MIMO transmission mode can reduce the error rate of the system by using the diversity gain of multiple antennas and can improve the capacity of the system by using the multiplex gain of multiple antennas. As shown in FIG. 1(B), in MU-MIMO transmission mode, the base station transmits multiple data flows on the same time frequency resource to multiple users, which may be considered as a space division multiple access (SDMA) technique where data is transmitted for multiple users, thereby improving the capacity of the system.
Pre-coding technique is a signal processing technique which pre-processes the symbols to be transmitted by using channel state information at the transmitting end so as to cancel interference and improve the capacity of the system. The pre-coding technique may be classified as two types, i.e. codebook-based pre-coding method and none-codebook-based pre-coding method. To utilize the pre-coding technique, the transmitting end needs to obtain the information about the transmission channel and to determine a pre-coding matrix based on the information about the transmission channel. In multiple input multiple output system, when the base station uses the pre-coding technique for downlink transmission, it requires the user equipment (UE) to feed back downlink channel information. Channel information feedback may be performed by using explicit channel information feedback mode, implicit channel information feedback mode or a feedback mode using the symmetric features of TDD channel.
In explicit channel information feedback, the downlink channel information is fed back directly, and in implicit, the channel information is returned implicitly by feeding back a pre-coding matrix or the like.
LTE (Long Term Evolution) proposes an implicit channel information feedback mode. In LTE, the user equipment selects a pre-coding matrix based on the transmission mode and the measured parameters of downlink channel, and feeds back a pre-coding matrix indicator (PMI) corresponding to the pre-coding matrix to the base station. For example, if the SU-MIMO mode is used, the user equipment returns to the base station a PMI of the pre-coding matrix suitable for the SU-MIMO mode, and if the MU-MIMO mode is used, the user equipment returns to the base station a PMI of the pre-coding matrix suitable for the MU-MIMO mode.